BMI formula: the calculation explained step by step

The Body Mass Index formula is short enough to fit on one line, which is exactly why it became a standard. It needs two measurements anyone can take, no equipment beyond a scale and a tape measure, and no assumptions about age, sex or origin. That simplicity is both its strength and the source of every misunderstanding about it.

The formula

In words: divide your weight in kilograms by your height in metres multiplied by itself. The BMI calculator on this site applies exactly this, with no hidden adjustment.

A fully worked example

Take someone who weighs 70 kilograms and measures 1.75 metres. The calculation runs in two steps.

Step one — square the height. 1.75 × 1.75 = 3.0625.

Step two — divide the weight by that result. 70 ÷ 3.0625 = 22.857…, which is reported as 22.9.

On the BMI chart, 22.9 falls inside the normal weight band. Notice that the intermediate value is not rounded: rounding 3.0625 to 3.1 before dividing would give 22.6, a visible drift from a single premature rounding.

Calculating in pounds and inches

There is no need to memorise a separate imperial formula. Convert both measurements first, then apply the same division. One inch equals 2.54 centimetres exactly, and one pound equals approximately 0.4536 kilograms.

Someone who is 5 feet 9 inches tall has a height of 69 inches, which is 69 × 2.54 = 175.26 centimetres, or 1.7526 metres. Someone weighing 154 pounds weighs 154 × 0.4536 ≈ 69.9 kilograms. Dividing 69.9 by 1.7526 squared gives roughly 22.8 — the same territory as the metric example above, as it should be, since it is nearly the same person.

Converting rather than switching formula has a practical advantage: there is only one calculation to check when a result looks wrong.

The mistakes that produce wrong numbers

Rounding the height

Height enters the formula squared, so an error there counts twice over. Rounding 1.72 to 1.70 shifts the final index by more than most people expect. Measure without shoes, standing straight, and keep the centimetre.

Using an old weight

The index describes a moment, not a history. Using a weight from several months ago produces a number that describes a body that no longer exists.

Rounding too early

Keep full precision through the calculation and round only the final result, to one decimal place. Rounding at each step compounds the drift, as the worked example above shows.

What the formula does not include

Reading the equation makes its blind spots obvious: there is no term for muscle, none for bone density, none for where fat is stored, and none for age or sex. Nothing in the calculation can distinguish a kilogram of muscle from a kilogram of fat, because the only thing it receives is total mass.

That is not a flaw in the arithmetic — it is the deliberate trade-off that made the index usable at population scale. But it means the result should be read as one indicator among several. The limitations of the Body Mass Index set out where the number stops being informative and what to look at alongside it.

When to seek advice

The calculation itself is never a reason to consult anyone. What matters is what it suggests, and whether it matches how you feel. If your weight has changed without a deliberate change in habits, or if the result concerns you, a health professional can interpret it alongside everything the formula cannot see.